The modern soaking pit has been developed to provide uniform heating of ingots to the desired temperature with a minimum of over-heating of the surface. In most modern designs, this is accomplished with automatic controls. The normal range for heating ingots is about 2400.degree.-2500.degree. F. The proper temperature level varies with grades of steel and sizes of ingots and characteristics of the rolling mill. Low-speed mills with many passes require the higher level of heating for certain grades of steel. Soaking pits serve the dual function of heating and acting as a reservoir to correct irregularities in the flow of ingots between the steel melting shop and the primary rolling mills. Briefly, soaking pits are deep chambers, or furnaces, of square, rectangular or circular shape, into which ingots are placed in an upright position through an opening at the top. A removable cover closes the pit opening. A series of pits, installed usually in rows are placed under cover of a building adjacent to the entering side of the blooming or slabbing mill to be served. The top of the pit is usually several feet above ground level. The pits are spanned by one or more electrically operated traveling cranes equipped with a traveling hoist for charging the ingots into the pits for lifting them out as they are needed by the mill.
The soaking pits are usually lined with a refractory material, such as alumina bench brick. The bottom, or floor, of the soaking pit is subjected to the greatest wear due to the weight of ingots, scale buildup, and localized heating from the ingots being treated, as well as puddling from molten metal. General practice has been to protect the soaking pit bottom with a granular material which soaks up slag, molten scale and molten metal and prevents these materials from directly contacting the refractory lining of the pit bottom. In "wet bottom" practice coke-breeze, a by-product from the coke ovens of the steel mill, has been typically used as a bedding material, or "bottom-making" material.
Where coke breeze is used as a bottom-making material, pit bottoms are made up with coke breeze to a depth of approximately 12 inches to 16 inches. Ashes from burned breeze, or breeze contaminated with scale, refractory or other material, are removed through cinder holes, of which there are usually two, located in the bottom of the pit. Bottoms for the older pit designs generally are made up each day; in modern pits they are made up only every 5-7 days under normal conditions. A recent trend is to utilize what is known as a dry-bottom practice, in which dolomite or magnesite is used as a 2-inch to 4-inch thick covering on the pit hearth.
In "dry bottom" practice, loose refractory crushed brick, magnesite or dolomite is typically used as the bedding material. Such materials are considerably more expensive than coke breeze, but the operation of the "dry bottom" soaking pit may continue for up to five months. When these "dry bed" soaking pits have their bedding material changed, the unit is closed down, and completely cooled, the total downtime being about a week. There is a growing need to replace magnesite and dolomite used in soaking pits because the materials are increasing in cost due to shortages. The amount of bedding material required in a typical soaking pit is about 10-20 tons, determined by the operating area to be covered and the depth of the bedding material.
This invention is directed to the use of heat resistant olivine as a replacement for coke breeze and other bedding materials in soaking pits for steel ingots, and particularly, for "wet-bottom" soaking pits. There are many advantages obtained by replacing coke breeze with olivine. The most important of these is the substantially improved bed life. Typical bed life in "wet bottom practice" using coke breeze as the bedding material is 3-7 days. Olivine used in the same soaking pit has typically provided a bed life of about forty days, and in some cases up to fifty-nine days. Another important advantage of using olivine is that it can be reused after removal from the soaking pit. The recovered olivine can be used in a blast furnace as a stabilizer additive as described in U.S. Pat. No. 4,066,443 issued to James W. Currier on Jan. 3, 1978, and commonly assigned herewith.